Viral capsids assemble inside infected cells through a staged series of structural transformations. The initial structure that is assembled is seldom the biologically active form. To gain insight into this process considerable effort has been put into characterizing the structures of the initial and final states of the capsids. A missing piece of the picture is a characterization of the dynamic pathway of transformation. As a class, the dsDNA containing viruses such as the bacteriophage and Herpesviridae initially assemble a procapsid into which the DNA is packaged. DNA packaging requires an ATP dependent motor protein termed a portal or connector protein, and the process of packaging itself results in a structural transformation from an unstable to stable capsid. In this proposal we will obtain dynamic information about the process of DNA packaging and capsid expansion. The morphogenetic similarities of these dsDNA viruses allow us the luxury of selecting the system best suited for studying each step with the reasonable expectation that we will be discovering underlying general principles. Therefore, we will study the structural transformation accompanying DNA packaging in bacteriophage P22 and the portal or connector motor protein complex function in bacteriophage Phi-29. The specific aims of the proposal are: 1) To determine if there is a conserved structural motif in the portal protein motor complexes of these dsDNA viruses. 2) To characterize the motions that enable the P22 procapsid to capsid transformation. and 3) To identify the dynamic motions in the Phi-29 portal motor complex during DNA packaging. These studies will elucidate the fundamental mechanisms underlying the assembly of DNA viruses and may suggest novel targets for the development of antivirals.